by Eric Lyman and John Bacon, USA TODAY

by Eric Lyman and John Bacon, USA TODAY

VATICAN CITY - The conservative pope who brought tweeting to the Vatican stunned the Roman Catholic world Monday, becoming the first pope in 600 years to resign as leader of the 1 billion-member church.

Pope Benedict XVI, 85, cited age and declining health in his announcement, issued in Latin during a meeting of cardinals. The resignation, effective Feb. 28, means a conclave of cardinals will be held here next month to elect a successor.

"After having repeatedly examined my conscience before God, I have come to the certainty that my strengths due to an advanced age are no longer suited" for the task, he said.

"Both strength of mind and body are necessary, strengths which in the last few months have deteriorated in me to the extent that I have had to recognize my incapacity to adequately fulfill the ministry entrusted to me," he said.

The Rev. Federico Lombardi, a Vatican spokesman, said the process of selecting a pope will begin March 1. He said an exact date for the conclave to begin had not been determined.

"Before Easter (March 31), we will have the new pope. (Resigning) is not a decision he has just improvised," Lombardi said. "It's a decision he has pondered."

In Washington, President Obama praised Pope Benedict XVI and said he "warmly remembers" their meeting in 2009. "On behalf of Americans everywhere, Michelle and I wish to extend our appreciation and prayers," Obama said in a statement.

But not all Americans have been thrilled with Benedict's rule. His handling of sex abuse issues has drawn criticism from some victims' advocates.

"When forced to, he talks about the crimes but ignores the cover-ups, uses the past tense as if to suggest it's not still happening," said David Clohessy, the executive director of the Survivors Network of Those Abused by Priests. "He has vast powers and he's done very little to make a difference."

The pope's brother, Georg Ratzinger, talking from his home in Regensburg, Germany, said the pontiff had been advised by his doctor not to take any more transatlantic trips and had been considering stepping down for months.

Ratzinger, 89, said his brother was having increasing difficulty walking and his resignation was part of a "natural process."

High-level Vatican officials will continue to conduct the business of the church until a new pope is selected. The most important tasks of the pope cannot be delegated - teachings on faith and appointing bishops.

A conclave of cardinals under the age of 80 votes on the pope. The conclave kicks off with a Mass at St. Peter's Basilica, then the cardinals gather in the Sistine Chapel. The doors are locked - "conclave" comes from the Latin for "with a key" - before meditation, prayer and voting.

If no one receives the required two-thirds of the vote, the ballots and notes are burned and a chemical is added to make the smoke appear black. The cardinals return the next day and try again, maybe a few more times.

Finally, white smoke signifies a pope has been elected. Benedict was selected on the fourth ballot.

The last pope to resign was Pope Gregory XII, who stepped down in 1415 in a deal to end a split in the church.

Born Joseph Ratzinger in Germany in 1927, Benedict entered the seminary at the age of 12. He has said he was unable to avoid becoming a reluctant member of the Hitler Youth. In 1943, he was drafted into the anti-aircraft defense and was captured by American forces, spending several months as a prisoner of war.

He became pope in 2005 after the death of John Paul II. Benedict led the church through much of the priest abuse tragedies, and went on to become the first social media pope, tweeting in seven languages. His personal Twitter account, @pontifex, had nearly 3 million followers within three weeks of launching in December.

Church historian Matthew Bunson said it was announced that the pope will retire to Castel Gandolfo, the papal summer retreat south of Rome, then to a small monastery where some contemplative nuns live on the Vatican grounds.

Cardinal Timothy Dolan of New York, president of the U.S. Conference of Catholic Bishops, issued a statement lauding Benedict his eight years of "selfless leadership."

"The Holy Father brought the tender heart of a pastor, the incisive mind of a scholar and the confidence of a soul united with his God in all he did."

The announcement came as a surprise to many Catholics in Rome, though they were aware the pope was ill.

"My first impression was one of incredulity," said Stefano Marino, 73, an engineer and former director of technical services at the Vatican. One of Marino's tasks had been to help orchestrate the smoke signal that indicates a new pope is chosen. "On one hand, it has to be said he has shown courage by taking responsibility and making such a decision. ... But he has also showed a weakness that John Paul II did not have â?? John Paul II made a strength of failing health."

Several hundred people are gathered in St. Peter's Square under grey skies, praying. The curtains were drawn on the pope's apartment on the southern side of the square.

"I was coming here anyway, to pray, I decided to stay longer when I heard the news," said Maria Rosa Martino, 35, a shopkeeper working near the Vatican. "I hope he is not in pain, suffering. He is a holy man. He would not make this decision if he could carry on. I pray for his strength."

Antonio Fiorella, 51, an off-duty security guard, sad he was saddened by the news. "We will not have another pope with a greater faith than Pope Benedict," he said. "I don't know what will happen next."